The unavailability of acceptable human donor organs, the low rate of long term success due to host versus graft rejection, and the serious risks of infection and cancer are the main challenges now facing the field of tissue and organ transplantation. Because the demand for acceptable organs exceeds the supply, many people die each year while waiting for organs to become available. To help meet this demand, research has been focused on developing alternatives to allogenic transplantation. Dialysis is available to patients suffering from kidney failure, artificial heart models have been tested, and other mechanical systems have been developed to assist or replace failing organs. Such approaches, however, are quite expensive. The need for frequent and periodic access to dialysis machines greatly limits the freedom and quality of life of patients undergoing such therapy.
Xenograft transplantation represents a potentially attractive alternative to artificial organs for human transplantation. The potential pool of nonhuman organs is virtually limitless. Pigs are considered the most likely source of xenograft organs. The supply of pigs is plentiful, breeding programs are well established, and their organ size and physiology are compatible with humans. Therefore, xenotransplantation with pig organs offers a potential solution to the shortage of organs available for clinical transplantation.
Host rejection of such cross-species tissue remains a major concern in this area. The immunological barriers to xenotransplantation have been, and remain, formidable. The first immunological hurdle is “hyperacute rejection” (HAR). HAR is defined by the ubiquitous presence of high titers of pre-formed natural antibodies binding to the foreign tissue. The binding of these natural antibodies to target epitopes on the donor organ endothelium is believed to be the initiating event in HAR. This binding, within minutes of perfusion of the donor organ with the recipient blood, is followed by complement activation, platelet and fibrin deposition, and ultimately by interstitial edema and hemorrhage in the donor organ, all of which cause failure of the organ in the recipient (Strahan, et al. (1996) Frontiers in Bioscience 1, pp. 34-41).
Some noted xenotransplants of organs from apes or old-world monkeys (e.g., baboons) into humans have been tolerated for months without rejection. However, such attempts have ultimately failed due to a number of immunological factors. Even with heavy immunosupression to suppress HAR, a low-grade innate immune response, attributable in part to failure of complement regulatory proteins (CRPs) within the graft tissue to control activation of heterologous complement on graft endothelium, ultimately leads to destruction of the transplanted organs (Starzl, Immunol. Rev., 141, 213-44 (1994)). In an effort to develop a pool of acceptable organs for xenotransplantation into humans, researchers have engineered animals that produce human CRPs, an approach which has been demonstrated to delay, but not eliminate, xenograft destruction in primates (McCurry, et al., Nat. Med., 1, 423-27 (1995); Bach et al., Immunol. Today, 17, 379-84 (1996)).
In addition to complement-mediated attack, human rejection of discordant xenografts appears to be mediated by a common antigen: the galactose-α(1,3)-galactose (gal-α-gal) terminal residue of many glycoproteins and glycolipids (Galili et al., Proc. Nat. Acad. Sci., (USA), 84, 1369-73 (1987); Cooper, et al., Immunol. Rev., 141, 31-58 (1994); Galili, et al., Springer Sem. Immunopathol, 15, 155-171 (1993); Sandrin, et al., Transplant Rev., 8, 134 (1994)). This antigen is chemically related to the human A, B, and O blood antigens, and it is present on many parasites and infectious agents, such as bacteria and viruses. Most mammalian tissue also contains this antigen, with the notable exception of old world monkeys, apes and humans. (see, Joziasse, et al., J. Biol. Chem., 264, 14290-97 (1989). Individuals without such carbohydrate epitopes produce abundant naturally occurring antibodies (IgM as well as IgG) specific to the epitopes. Many humans show significant levels of circulating IgG with specificity for gal-α-gal carbohydrate determinants (Galili, et al., J. Exp. Med., 162, 573-82 (1985); Galili, et al., Proc. Nat. Acad. Sci. (USA), 84, 1369-73 (1987)). The α-galactosyltransferase (α-GT) enzyme catalyzes the formation of gal-α-gal moieties. Research has focused on the modulation or elimination of this enzyme to reduce or eliminate the expression of gal-α-gal moieties on the cell surface of xenotissue.
The elimination of the α-galactosyltransferase gene from porcine has long been considered one of the most significant hurdles to accomplishing xenotransplantation from pigs to humans. Two alleles in the pig genome encode the α-GT gene. Single allelic knockouts of the α-GT gene in pigs were reported in 2002 (Dai, et al. Nature Biotechnol., 20:251 (2002); Lai, et al., Science, 295:1089 (2002)).
Recently, double allelic knockouts of the α-GT gene have been accomplished (Phelps, et al., Science, 299: pp. 411-414 (2003)). WO 2004/028243 to Revivicor Inc. describes porcine animal, tissue, organ, cells and cell lines, which lack all expression of functional α1,3 galactosyltransferase (α1,3-GT). Accordingly, the animals, tissues, organs and cells lacking functional expression of α1,3-GT can be used in xenotransplantation and for other medical purposes.
PCT patent application WO 2004/016742 to Immerge Biotherapeutics, Inc. describes α(1,3)-galactosyltransferase null cells, methods of selecting GGTA-1 null cells, α(1,3)-galactosyltransferase null swine produced therefrom (referred to as a viable GGTA-1 null swine), methods for making such swine, and methods of using cells, tissues and organs of such a null swine for xenotransplantation.
One of the earliest known xenoantigens other than gal-α-gal is an epitope that Hanganutiu Deicher antibodies recognize, and which have long been associated with serum disease. The epitope has been identified as N-glycolylneuraminic acid (Neu5Gc), a member of the sialic acid family of carbohydrates. Among carbohydrates, sialic acids are abundant and ubiquitous. Sialic acid is a generic designation used for N-acylneuraminic acids (Neu5Acyl) and their derivatives. N-Acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) are two of the most abundant derivatives of sialic acids.
The Neu5Gc epitope is located in the terminal position in the glycan chains of glycoconjugates. Due to this exposed position, it plays an important role in cellular recognition, e.g. in the case of inflammatory reactions, maturation of immune cells, differentiation processes, hormone-, pathogen- and toxin binding (Varki, A., Glycobiology, 2, pp. 25-40 (1992)).
Glycoconjugates containing Neu5Gc are immunogenic in humans. In healthy humans, Neu5Gc is not detectable, although Neu5Gc is abundant in most mammals. The lack of Neu5Gc in man is due to an exon deletion in the human gene that prevents the formation of functional enzyme (Chou, H. H., et al. Proc. Natl. Acad. Sci. (USA), 95, pp. 11751-11756 (1998); Irie, A., et al. J. Biol. Chem., 273, pp. 15866-15871 (1998)). Thus, Neu5Gc-containing glycoconjugates act as antigens and can induce the formation of antibodies. Historically, the antibodies have been referred to as Hanganutziu-Deicher (HD) antigens and antibodies (Hanganutziu, M., CR Soc. Biol. (Paris), 91, p. 1457 (1924); Deicher, H., Z. Hyg., 106, p. 561 (1926)). Hanganutziu-Deicher antigens are detectable in many human tumors (colon carcinoma, retinoblastoma, melanoma and carcinoma of the breast) as well as in chicken tumor tissues (Higashi, H., et al. Cancer Res., 45, pp. 3796-3802 (1985)). Although the amount of antigen in tumors is very small (usually less than 1% of the total amount of sialic acid, often in the range of from 0.01 to 0.1%), it is capable of inducing the formation of Hanganutziu-Deicher antibodies (Higashihara, T., et al., Int Arch Allergy Appl Immunol., 95, pp. 231-235 (1991)). This immunological reaction is a potential barrier to xenotransplantation of Neu5Gc-containing pig organs to humans.
The Neu5Gc epitope is formed by the addition of a hydroxyl group to the N-acetyl moiety of Neu5Ac. The enzyme that catalyzes the hydroxylation is CMP-Neu5Ac hydroxylase. Thus, the expression of the CMP-Neu5Ac hydroxylase gene determines the presence of the Neu5Gc epitope on cell surfaces. Purification studies of CMP-Neu5Ac hydroxylase in mammals have shown that it is a soluble, cytosolic oxygenase that is dependent on cytochrome b5 and cytochrome b5 reductase (Kawano, T., et al., J. Biol. Chem., 269, pp. 9024-9029 (1994); Schneckenburger, P., et al., Glycoconj. J., 11, pp. 194-203 (1994); Schlenzka, W., et al., Glycobiology, 4, pp. 675-683 (1994); Kozutsumi, Y., et al., J. Biochem. (Tokyo), 108, pp. 704-706 (1990); and, Shaw, L., et al. Eur. J. Biochem., 219, pp. 1001-1011 (1994)).
Another important feature of Neu5Gc is that it acts as an adhesion molecule for pathogens, allowing for entry into the cell (Kelm, S. and Schauer, R., Int. Rev. Cytol, 179, pp. 137-240 (1997)). This causes disease and economic losses in certain livestock species. Specifically, enterotoxigenic Escherichia coli with K99 fimbriae infect newborn piglets by binding to Neu5Gc in gangliosides such as Nue5Gca2→3Galβ1→4Glcβ1→1′ ceramide [GM3(Neu5Gc)], N-glycolylsialoparagloboside and GM2(Neu5Gc) attached to intestinal absorptive and mucus secreting cells, causing a potentially lethal diarrhea (Malykh, Y., et. al., Biochem. J., 370, pp. 601-607 (2003); Kyogashima, M., et al., (1993); Teneberg, S., et al., FEBS Letters, 263, pp. 10-14 (1990); Isobe, T., et al., Anal. Biochem., 236, pp. 35-40 (1996); Lindahl, M. and Carlstedt, I., J. Gen. Microbiol., 136, pp. 1609-1614 (1990); King, T. P., et al., Proceedings of the 6th International Symposium on Digestive Physiology in Pigs, pp. 290-293, (1994)). Pig rotavirus infects pig newborns causing diarrhea by binding to GM3(Neu5Gc). Pig transmissible gastroenteritis coronavirus infects pigs via entry into glycoconjugates containing α2,3-bound Neu5Gc (Schultz, B., et al., J. Virol., 70, pp. 5634-5637 (1996)).
CMP-Neu5Ac hydroxylase has been isolated from mouse liver and pig submandibular glands to homogeneity and characterized (Kawano, T., et al., J. Biol. Chem., 269, pp. 9024-9029 (1994); Schneckenburger, P., et al., Glycoconj. J., 11, pp. 194-203 (1994); and, Schlenzka, W., et al., Glycobiology, 4, pp. 675-683 (1994)).
Schlenzka, et al. (Glycobiology, Vol. 4, pp. 675-683 (1994)) purified the enzyme from pig submandibular glands using ion exchange chromatography, chromatography with immobilized triazin dyes, hydrophobic interaction chromatography and gel filtration. Schneckenburger et al. (Glycoconj. J., Vol. 11, pp. 194-203 (1994)) isolated the CMP-Neu5Ac hydroxylase from mouse liver. Both the CMP-Neu5Ac hydroxylase from pig submandibular glands and the one from mouse liver are soluble monomers having a molecular weight of 65 kDa. Their catalytic interactions with CMP-Neu5Ac and cytochrome b5 are very similar to one another. The activity of these enzymes seems to be dependent on an iron-containing prosthetic group.
JP-A 06 113838 describes the protein and DNA sequences of murine CMP-Neu5Ac hydroxylase, as well as a monoclonal antibody that specifically binds to the hydroxylase.
PCT Publication No. WO 97/03200A1 to Boehringer Manheim GMBH discloses a partial cDNA for the porcine CMP-Neu5Ac hydroxylase. This application discloses a cDNA sequence beginning in the middle of Exon 8 of the CMP-Neu5Ac hydroxylase gene (further disclosed as GenBank accession number Y15010).
Martensen, L., et al. (Eur. J. Biochem., Vol. 268, pp. 5157-5166 (2001)) discloses a full length amino acid sequence of porcine CMP-Neu5Ac hydroxylase.
PCT Publication No. WO 02/088351 to RBC Biotechnology discloses a partial cDNA and genomic sequence (exons 7-11 as well as partial genomic sequence surrounding each exon) of porcine CMP-NeuAc hydroxylase. In addition, methods are provided to generate porcine cells and animals lacking the CMP-NeuAc hydoxylase epitope, optionally, in combination with other genetic modifications, such as inactivation of the alpha-1,3-galactosyltransferase gene and/or insertion of complement proteins.
It is an object of the present invention to provide genomic and regulatory sequences of the porcine CMP-Neu5Ac hydroxylase gene.
It is an object of the present invention to provide the full length cDNA, as well as novel variants of the CMP-Neu5Ac hydroxylase gene.
It is another object of the invention to provide novel nucleic acid and amino acid sequences that encode the CMP-Neu5Ac hydroxylase gene.
It is yet a further object of the present invention to provide cells, tissues and/or organs deficient in the CMP-Neu5Ac hydroxylase gene.
It is another object of the present invention to generate animals, particularly pigs, lacking a functional CMP-Neu5Ac hydroxylase gene.
It is yet a further object of the present invention to provide cells, tissues and/or organs deficient in the CMP-Neu5Ac hydroxylase gene for use in xenotransplantation of non-human organs to human recipients in need thereof.